Distribution transformers have to show good resistance to mechanical and thermal stress that is associated on one hand with overload and short circuit conditions, and on the other hand with adverse climatic conditions. For said applications dry-type transformers with epoxy-insulated coil windings, reinforced and bound together by roved glass fibers, are state of the art. Such transformers are commercially available from ABB under the trademark RESIBLOC®. A high glass-fiber content (approximately 70% in weight) is instrumental in reaching the required excellent mechanical strength and thermal-shock stability at both high and low temperature values. Epoxy-resin insulated coils according to the RESIBLOC® technique are manufactured by alternately applying a winding layer and a layer consisting of continuous glass fibers. Said fibers are dipped into a liquid epoxy resin bath and are subsequently wound onto the coil windings. The liquid epoxy resin fills the voids between the coil windings. After completing the coil-winding process the liquid epoxy resin is cured in an oven and becomes solid.
State-of-the-art epoxy-insulated transformers show an outstanding product performance. Their manufacture, however, is rather labour- and cost-intensive. The excess of epoxy resin resulting from applying the wetted fibers has to be spread on the coil or removed manually by an operator.
To be suitable for the fiber roving technique, the used epoxy resin has to be of comparably low viscosity. For maintaining a uniform resin distribution in the final curing step, the coil has to be tilted back and forth until gelling and solidifying of the epoxy resin insulation matrix has taken place. Without this tilting the epoxy resin would drip off and accumulate at a bottom end of the coil. The complex curing cycle lasts for more than 10 hours and cannot be shortened easily.
The two above-mentioned reasons lead to significant manufacturing costs. In addition the manual application of the liquid epoxy-resin/anhydride prepolymer mixture in an open wet process can lead to significant exposure of workers to toxic chemical compounds. Furthermore for environmental reasons unused uncured epoxy resin has to be disposed off appropriately.
EP 1 211 052 A1 shows a process and an apparatus for bandaging bodies with fiber-reinforced plastic bands. The fiber-reinforced thermoplastic band is unspooled and is fed, via a fusing apparatus, to a pressing apparatus. The thermoplastic polymer band is melted superficially and is subsequently pressed onto the body to be bandaged.
U.S. Pat. No. 5,954,909 discloses a direct adhesive process for adhering conductors to supports and for adhering a further wire conductor layer onto an adhesive film applied to an existing wire conductor layer. The adhesive is painted or sprayed onto a mold release form, wherein a fiber mat can be incorporated or fiber particles can be embedded. The adhesive layer can be B-staged by thermal treatment, thus forming a 2-D adhesive sheet. For coil production, the adhesive sheet is then arranged patch-wise on a 2-D conductor wire area in a separate, discontinuous placement step. In a subsequent step, heat treatment and over-wrapping is used to bond and press the adhesive sheet onto the existing 2-D conductor layer.
DE 1 540 133 discloses a method for producing coil isolations using a resin tape and a separate hardener tape. At least one tape may comprise mica or glass fibers. The separate tape approach is chosen as an alternative to prepreg tapes in order to improve storage times. In a first step, a coil isolation is produced by winding the combination of resin tape and hardener tape onto a conductor layer. In a second step, the coil is then heated for 1-3 hours for hardening the resin. This hardening process is performed in a separate, discontinuous production step.